In the television industry, it is common to transmit video signals in one of several standard digital formats between camera, studio, transmitting, and other equipment. Coaxial cable having nominal 75 Ohm impedance is commonly used to make these connections.
A serial TV signal contains a large amount of very high frequency energy. In the case of component (as opposed to NTSC) transmission, color television waveforms are digitized, usually to 10 bit accuracy, at a rate of 13.5 MHz for the Y (luminance) component and a rate of 6.75 MHz for each of the R-Y and B-Y (color difference) components. When the samples are interleaved as R-Y, Y, B-Y, Y, this results in a final multiplexed 10 bit parallel rate of 27 megabytes per second. Subsequent conversion of the 10-bit data to serial format results in a bit rate of 270 Mb/s. It should be noted that other serial standards exist and currently result in bit rates of 143 Mb/s for NTSC composite, 177 Mb/s for PAL composite and 360 Mb/s for wide-screen component signals.
Coaxial cables of the type commonly used for the transmission of serial digital television signals (such as Belden type 8281) are of high quality, but nevertheless have a large loss at the typical 270 Mb/s transmission rate. At this data rate, the attenuation is approximately 33 dB for 300 meters of cable.
Connecting equipment is considered acceptable by those in the broadcast industry if it can operate fault-free with 300 meters of cable. Unfortunately, practical prior art equipment operating in real-world conditions typically operates fault-free with a maximum cable length of 150 to 250 meters.
Good input return loss, on the order to 20 dB, is desirable because poor return loss causes reflections on the associated coaxial input cable. Such reflections can produce signal cancellation effects, thus rendering the signal unusable, especially with short (e.g. 20 meter) cables. Prior art equipment has a return loss often in the range of 8 dB to 18 dB. Current standards call for a minimum of 15 dB.
In the prior art, interface circuits known as cable equalizers were provided to connect a serial digital input signal to the coaxial cable for video signal transmission. In conventional cable equalizers, the receiving end of the cable is connected to an integrated circuit containing an automatically-adjusted cable response-loss equalizing circuit. The purpose of this circuit is to restore the high-frequency components of the waveform to normal levels. Once the waveform is restored by the equalizer, further processing, usually digital, of the incoming signal can proceed without bit errors created by reflections on the cable. However, the signal processing performed by the cable equalizers results in a loss of signal strength.
FIG. 1 is a functional block diagram of a GENLINX.TM. GS9004A serial digital cable equalizer integrated circuit manufactured by Gennum Corporation, Burlington, Ontario, Canada. This device, shown generally at 100, is used to equalize video signals from a coaxial cable, and is implemented as a 14 pin chip powered by a single 5 VDC power supply. The chip is capable of operating at up to 400 Mb/sec. A serial digital input signal is connected to input 102 of integrated circuit 100, either differentially or single ended, with the unused input being decoupled. The equalized signal is generated by passing the cable signal through a voltage variable filter 104 having a characteristic which closely matches the inverse cable loss characteristic. Additionally, the variation of the filter characteristic with control voltage is designed to imitate the variation of the inverse cable loss characteristic as the cable length is varied. The amplitude of the equalized signal is monitored by a peak detector circuit 106 which produces an output current with polarity corresponding to the difference between the desired peak signal level and the actual peak signal level. This output is integrated by an external AGC filter capacitor 108, providing a steady control voltage for voltage variable filter 104 through filter control 110.
Signal strength indicator output 112 provides a level proportional to the amount of AGC. As the filter characteristic is varied automatically by the application of negative feedback, the amplitude of the equalized signal is kept at a constant level which represents the original amplitude at the transmitter. The equalized signal is then DC restored by DC restorer 114, which restores the logic threshold of the equalized signal to its correct level irrespective of shifts due to AC coupling.
In the final stage of signal conditioning within the Gennum integrated circuit 100, a comparator 116 converts the analog output of the DC restorer to a regenerated digital output signal having pseudo-ECL voltage levels. An output eye monitor 118 allows verification of signal integrity after equalization, prior to reslicing.
FIG. 2 shows a conventional operational test setup circuit for integrated circuit 100 as disclosed in the Gennum data sheets. As can be seen, the cable bearing the input signal is typically capacitively coupled directly to serial digital input 102 without any intervening processing or amplification circuitry. A 75 ohm resistor ties the input to ground to provide the desired input impedance. The Gennum integrated circuit 100 is used by a number of manufacturers in making serial digital cable equalizing circuits, but as far as the inventor is aware, this circuit has not been used in conjunction with an analog preamplification and input processing circuit. The high frequency impedance of the Gennum circuit has a substantial capacitive component.
FIG. 3 shows another known serial digital cable equalizer circuit, a 41212 encoder manufactured by Recherches Miranda, Inc. This circuit uses an SBX 1602A integrated circuit manufactured by Sony Corporation of Japan. As in the case of the Gennum circuit shown in FIG. 2, the circuit in FIG. 3 has a cable input 302 which is capacitively coupled to the input of the integrated circuit. A resistor R33 is also provided to create the desired input impedance.
The industry has generally accepted that there is an unavoidable tradeoff required between the competing goals of maintaining high signal strength to drive a long cable, and minimizing cable reflections which cause bit errors or "snow" in the picture. Modern television sets and cable television transmission systems are capable of creating a high quality picture, and consumers have become accustomed to receiving pictures without interference or signal errors. While any transmission system may create an occasional error, the occurrence of any significant number of bit errors prior to transmission to the receiving stations is considered unacceptable. Integrated circuits designed for this purpose, such as the Gennum circuit described above with reference to FIGS. 1 and 2, have a variable internal automatic gain control, but the provision of this amplification does not overcome the inherent tradeoff between maintaining high signal strength and minimizing cable reflections.
Prior art efforts to increase return loss (that is, minimize cable reflections) beyond a certain level created a substantial loss of signal amplitude, resulting in a large number of errors with long cables. Thus, circuits of the type shown in FIGS. 1-3 are limited in their ability to drive long cables without introducing substantial error. Amplification of the signal within the digital circuit is ineffective because any existing cable reflection signal components are also inherently amplified, negating potential gains from increasing the return loss on the front end. A less-than-satisfactory compromise was usually made between achieving reasonably good input return loss (15 dB being a minimum target) and error-free overall performance (300 meters minimum with less than 1 error per day).
While not generally recognized as a major source of the problem by those skilled in this art, practical equipment designs (using available integrated circuits to receive the signal) have suffered from certain additional high-frequency losses which reduce performance with long cables. These losses usually arise from the need to be able to plug the circuit boards into some type of chassis-mounted backplane which would in turn provide a mount for coaxial connectors for the incoming cable. For reason of cost, economical connectors are used which are neither of a coaxial nature nor designed to carry signals having such a large high-frequency content. In attempts to correct the resultant impedance discontinuity, additional signal losses are incurred. The effect of this loss is compounded because the available integrated circuit equalizers often have marginal equalization gain performance. The cumulative result is that it has been difficult to achieve satisfactory performance with more than 250 meters of cable in equipment having plug-in circuit boards.
The inventor has discovered through his study of the problem that the error rate begins increasing at a substantial rate when the cable length is extended beyond a certain threshold limit imposed by the capability of the cable equalizer circuit. Typically, with circuits of the type shown in FIGS. 1-3, this threshold occurs with about 250 meters of cable. Near the threshold, adding a small amount of cable length, such as 30 meters, can completely degrade the picture being transmitted. Because of the existence of this threshold effect, and because the threshold point may vary due to temperature and other factors, it is generally desirable to provide an equalizing circuit capable of driving a good deal more cable than is being used, to provide a safety factor.
Thus, there are a number of problems and conflicting requirements facing designers of serial digital equalizing circuits, and there is a need for an improved serial digital equalizing circuit that overcomes the problems and limitations experienced in the prior art.